BEIRUT – Doctors Without Borders pleaded on Monday for access to treat the wounded in the rebel-held part of Syria's Aleppo as government forces pressed ahead with an offensive that has killed hundreds of people in recent weeks.

The international charity, also known by its French acronym MSF, said in a statement that medical workers in Aleppo are exhausted and that the overstretched facilities face an impending fuel shortage. MSF, which supports eight hospitals in Aleppo's besieged eastern quarters, says just 35 doctors remain in the area, serving a population of 275,000.

Eastern Aleppo's Health Directorate said the wounded were sleeping outside overcrowded hospitals, waiting for care. The U.N. has warned that the Aleppo bombardment by Syrian and Russian warplanes could leave thousands more dead by the year's end.

"Russia and Syria must stop the indiscriminate bombing now and abide by the rules of war to avoid the extreme suffering of the unprotected civilian population," said Pablo Marco, MSF's operations manager for the Middle East.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict through local contacts, reported heavy fighting along the east-west Aleppo front lines. Another activist-run group, the Local Coordination Committees, said rebels were fighting to repel government forces from the city's largest water facility, which serves over a million people.

In another besieged area near the capital, Damascus, doctors reported up to two dozen cases of kidney failure that they said resulted from malnutrition. Muhammad Darwish, a local physician, said doctors confirmed renal failure in 12 people in the town of Madaya and were investigating another 12 cases.

Government forces have laid siege to Madaya, home to some 40,000 people, since late last year. Last winter, MSF reported at least 16 deaths there resulting from malnutrition and lack of medical care.

"We are only eating carbohydrates. We aren't receiving any vitamins or protein," said Darwish.

The government has prohibited the U.N. from delivering seeds or dialysis kits to the town, in what the opposition says is a strategy aimed at forcing the town to surrender.

U.N. Secretary-General Ban Ki-moon blamed Syrian President Bashar Assad for the bloodshed, saying more than 300,000 people have been killed since the start of the war because of his "failure of leadership."

In an interview with the German broadcaster Deutsche Welle, Ban referenced previous conflicts in which the international community failed to halt mass killings.

"We should have prevented Srebrenica. We should have prevented the Rwandan genocide. In Aleppo, we're doing our utmost efforts," he said. "The future of one person, like President Assad, should not block this process."

The U.N. Security Council is deadlocked over how to respond to the Aleppo crisis, with Russia on Saturday vetoing a French-drafted resolution demanding an immediate halt to Russian and Syrian airstrikes and the rest of the council voting down a Russian resolution calling for the evacuation of opposition fighters.

Moscow has been a key backer of Syrian President Bashar Assad throughout the conflict.

Russia and Syria's government have both endorsed a proposal by U.N. envoy Staffan de Mistura to evacuate Al Qaeda-linked militants from east Aleppo while skirting around the reciprocal part of the proposal, to abide by a cease-fire for the besieged area.

An official at Syria's Foreign Ministry said the government would ensure the safety of any civilians or militants who desire to leave, saying gunmen could take their personal weapons and relocate to other rebel areas. The official spoke to the AP on condition of anonymity in line with ministry regulations.

The Czech Republic meanwhile dispatched a plane with medical aid to the Syrian Arab Red Crescent. The Czech Republic has coordinated aid deliveries directly with the Syrian government, while other Western nations have worked through third parties. The Czechs pledged earlier this year to provide aid worth some $8 million to Syria until 2019.

Elsewhere in Syria, the Observatory said Monday that pro-government forces recaptured two towns north of the central city of Hama in a counteroffensive aimed at reversing a string of losses to ultraconservative rebels in the region.

After weeks of rebel advances outside the central city, which appeared aimed at severing the government's supply lines between Damascus and Aleppo, the leading force behind the offensive, Jund al-Aqsa, became embroiled in fighting with another powerful rebel group, paving the way for the government advance.

On Sunday, Jund al-Aqsa agreed to merge with the Al Qaeda-linked rebel faction in Syria, the Fatah al-Sham Front, as an attempt to defuse the conflict.